1. Field of the Invention
The present invention relates to a surface acoustic wave filter having a plurality of SAW resonators arranged on a piezoelectric substrate and, more particularly, to a surface acoustic wave filter having a plurality of SAW resonators arranged so as to define a ladder type filter circuit.
2. Description of the Related Art
A surface acoustic wave filter having a plurality of SAW resonators arranged on a piezoelectric substrate is known, for example, as a high-frequency band filter for use in mobile communication apparatuses. For example, Japanese Patent Publication (Kokoku) No. 56-19765 discloses a surface acoustic wave filter having a ladder type filter circuit including a plurality of SAW resonators on a piezoelectric substrate.
FIG. 11 is a schematic plan view for explaining a surface acoustic wave filter disclosed in the above-mentioned prior art.
A surface acoustic wave filter 51 includes a rectangular piezoelectric substrate 52, on which a plurality of SAW resonators are arranged to define series arm resonators 53 and 54 and parallel arm resonators 55 and 56. That is, the series arm resonators 53 and 54 are connected in series in a series arm between an input terminal 57 and an output terminal 58. Also, parallel arm resonators 55 and 56 are connected between the series arm and a ground potential. The series arm resonators 53 and 54 and the parallel arm resonators 55 and 56 are alternately disposed between the input and output terminals 57, 58.
In the surface acoustic wave filter 51, the series arm resonators 53 and 54 and the parallel arm resonators 55 and 56 are constructed in such a manner that interdigital transducers (hereinafter referred to as IDTs) 53a, 54a, 55a, and 56a are provided at the centers of the series and parallel arm resonators, respectively, and reflectors 53b and 53c, 54b and 54c, 55b and 55c, and 56b and 56c are provided on the opposite sides of the IDTs 53a-56a. That is, in the surface acoustic wave filter 51, each of the series arm resonators 53 and 54 and the parallel arm resonators 55 and 56 is comprised of a one-port SAW resonator having reflectors on the opposite sides of an IDT.
The principle of the operation of the above-described surface acoustic wave filter 51 is as described below.
Referring to FIG. 12 in which only electrode portions are schematically shown, a one port SAW resonator 60 has a structure in which reflectors 62 and 63 are placed on the opposite sides of an IDT 61 located at a center thereof. The IDT 61 has a structure in which a comb-shaped electrode 61a having at least one electrode finger and a comb-shaped electrode 61b having at least one electrode finger are arranged so that their electrode fingers interdigitate with each other.
In the one port SAW resonator 60, surface acoustic waves excited by the IDT 61 are reflected by the reflectors 62 and 63 to form standing waves confined between the reflectors 62 and 63. The SAW resonator 60 therefore operates as a resonator having a large Q value. As is well-known, in the impedance characteristics of the SAW resonator 60, a pole of a lower impedance exists in the vicinity of a resonant frequency fr while a pole of a higher impedance appears at an antiresonant frequency fa.
In the surface acoustic wave filter 51, a passband is obtained by utilizing the impedance characteristics of the one port SAW resonators described above. More specifically, the resonant frequencies fr of the series arm resonators 53 and 54 and the antiresonant frequencies fa of the parallel arm resonators 55 and 56 are set equal to each other to match the output and input impedances with the characteristic impedance at about the resonant and antiresonant frequencies which are equal to each other, thus setting a passband.
Because the one port SAW resonator has the above-described impedance characteristic, very high impedances result at the antiresonant frequencies of the series arm resonators 53 and 54 while very low impedances result at the resonant frequencies of the parallel arm resonators 55 and 56. In the surface acoustic wave filter 51, therefore, attenuation regions are defined with poles corresponding to these frequencies.
The surface acoustic wave filter 51 has the series arm resonators 53 and 54 and the parallel arm resonators 55 and 56 arranged as described above and, therefore, can be designed so as to reduce insertion loss and can achieve a comparatively large amount of attenuation in the vicinity of the passband.
Japanese Patent laid-Open Publication No. 5-183380 also discloses a surface acoustic wave filter arranged in the same manner as the surface acoustic wave filter 51 and has an inductance added to the parallel arm resonators to achieve a wide frequency range.
In recent years, there has been a strong demand for expanding transmitting and receiving frequency bands of mobile communication apparatuses such as portable telephones. As a result, widening of the frequency band has also been demanded with respect to band filters used in such apparatuses. Moreover, an increase in the amount of attenuation in rejection regions of band filters for use in such apparatuses is strongly demanded. That is, there is a need to obtain a large amount of attenuation in a receiving frequency band in a transmitting filter as well as in a transmitting frequency band in a receiving filter.
In band filters for mobile communication apparatuses, the frequencies of the above-mentioned rejection regions are in the vicinity of a passband. Surface acoustic wave filters having a ladder type circuit operate based on the above-described principle and, therefore, have a comparatively large amount of attenuation in the vicinity of a passband. Thus, surface acoustic wave filters having a ladder type circuit configuration have characteristic suitable for a band filter of mobile communication apparatuses.
Surface acoustic wave filters having a ladder type circuit achieve a very large amount of attenuation at an attenuation pole. However, since the frequency range of the attenuation pole is narrow, the amount of attenuation decreases abruptly with deviation from the frequency of the attenuation pole. That is, a large amount of attenuation is obtained at the frequency of the attenuation pole in a rejection region in the vicinity of a passband, but an increase in the frequency range in which the amount of attenuation is large is strongly demanded since the frequency range in which the amount of attenuation is large is comparatively narrow.
Conventionally, the method (1) of increasing the ratio of the capacitances of a parallel arm resonator and a series arm resonator and the method (2) of distributing a plurality of attenuation poles by using a plurality of parallel arm resonators and setting the resonant frequencies of the parallel arm resonators to be different from each other have been tried as a method for extending the frequency range of a large-attenuation portion in a rejection region of a surface acoustic wave filter having a ladder type circuit.
However, the method (1) of increasing the amount of attenuation by the capacitance ratio causes an increase in insertion loss, and the method (2) of setting different resonant frequencies of a plurality of parallel arm resonators causes a failure of impedance matching in a passband, which results in an increase in reflection loss.
That is, as long as the conventional methods for increasing the frequency band in which the amount of attenuation is large are used, there is a limit to the advantageous effect of such methods because of an increase in insertion loss and reflection loss in addition to a failure of impedance matching.